Leak detecting instruments are used in a variety of industries to determine whether products are properly manufactured and assembled. Leak detecting instruments are used to test products for the presence of leaks which degrade the performance of the product during the product's useful life. Not all leaks are fatal to the performance of a product and a maximum acceptable leak is often established for a product. Consequently, leak detecting instruments must be able to distinguish between leaks which fall above or below the maximum acceptable leak.
Because leak detecting instruments not only detect leaks but measure the amount of leakage as well, leak detecting instruments are calibrated to accurately measure the amount of leak from a part being tested. Leak detecting instruments are therefore calibrated to a leak standard providing a known amount of flow. The leak standard establishes a known flow rate which is used to calibrate the sensing apparatus of the leak detection equipment measuring leakage, thereby ensuring that the leak detection equipment accurately measures the amount of leak present in the parts being tested. Devices measuring flow rate are used to establish the leak standard.
One particular type of device used to measure flow rate is a soap film flowmeter. The soap film flowmeter consists of a precision glass tube with graduations marking specific volumes referenced to a zero point. The bottom portion of the tube is closed to create a reservoir containing a soap solution. The soap film flowmeter is connected to a flow control needle valve. A source of pressurized air or gas is connected to an input of the needle valve and the output is connected to an input located at the bottom portion of the soap film flowmeter. Pressurized air or gas is injected into the tube through the needle valve below the surface of the soap solution. As air flows into the tube, a bubble is formed and rises in the tube. Flow rate is determined by measuring the travel time of the bubble between the reference zero point and a known graduation mark. The volume traversed by the bubble is divided by the travel time to arrive at a flow rate.
Most leak detecting instruments are provided with a quick-connect fitting or calibration port in the test part pressurization line to facilitate insertion of a leak standard. Typically, an input of a flow control needle valve is attached to the quick-connect fitting. To set a calibrated leak using a soap film flowmeter, an outlet of the needle valve is connected to the soap film flowmeter. As air flows through the needle valve and into the flowmeter, a soap bubble is formed. The needle valve is adjusted until the resultant flow causes the bubble to traverse the tube for a period of time corresponding to the desired flow rate. In this way, the flow control needle valve provides a measured amount of flow for use as a leak standard.
While soap film flowmeters are used to establish a leak standard, soap film flowmeters have certain disadvantages. For instance, the accuracy of the measured flow rate is extremely dependent upon the operator using the device. Because the travel time of a bubble passing from one graduation to another indicates flow rate, the operator must observe when the bubble passes each graduation. If the operator misjudges when the bubble passes either graduation, the reading of flow is not accurate.
The soap film flowmeter must also be placed on a perfectly level surface to yield accurate measurements since the surface of the soap solution must be parallel to the graduation marks to yield accurate readings. In addition, the glass tube is an inherently fragile device requiring extra care which is not always easy to supply in the often hostile production environment.
Another type of device used to measure flow rate is a rotameter. Rotameters are variable area flowmeters which include a container having an input and output connection for holding a fluid into which a positionable restrictor is placed. The restrictor is displaced in the fluid by an applied flow of gas or air to the input. The restrictor moves to a position dictated by the gas flow-through. The position of the restrictor is dependant upon gravity or gravity supplemented with a spring resistance to provide essentially a constant differential pressure.
One type of rotameter is a tapered-tube rotameter in which gas flows upwardly through a tapered tube, lifting a shaped weight or float to a position where the upward fluid force just balances the float weight, thereby giving a float position that indicates flow rate. The position of the float is determined by ascertaining where the float is positioned within the tube, typically marked by graduation lines. The position of the float can also be electronically determined in situations where the float carries the core of an inductance pickup for transmission of the float position through a nonmagnetic tube seal.
The rotameter suffers some of the same disadvantages as the soap film flowmeter in that accurate readings are dependant upon the proper volume and surface level of the included liquid. In addition, the rotameter, like the soap film flowmeter, is an inherently fragile device requiring extra care due to its size and shape.
Fixed and variable orifices are also used to establish leak standards. The orifice is connected to the flowmeter and controls the rate of air flow measured by the flowmeter. Fixed orifices can only be used for a specific leakage value at a specified pressure. Calibration at other values requires the use of different fixed orifices. These orifices are fragile, nonrenewable, and subject to clogging from contamination. In particular, once the fixed orifice is calibrated by a rotameter or flowmeter, it is removed from the rotameter or flowmeter and used as the leak standard. After removal, the flow from the orifice is no longer continuously monitored, and any change in flow due to clogging or any other change in orifice size after proper adjustment goes undetected. Consequently, once the fixed or variable orifice is calibrated in this fashion, the operator cannot be certain that the leak standard remains at the same value as calibrated.
Variable orifices or needle valves can be used at various leak rates and pressure conditions, but each time the orifice setting is changed, the variable orifice must be recalibrated. This is a lengthy, complex process requiring skill and knowledge. Not only must an operator connect the variable orifice to the rotameter or flowmeter, the operator must exercise a great amount of caution in connecting the orifice to the rotameter or flowmeters, due to their fragileness. Once connected, the operator must make sure that the flowmeter is level, and that the timing of bubble flow or the reading of the level of the float is made accurately. Even after the orifice is calibrated, the operator has no way of knowing whether the calibrated flow through the orifice remains at the proper value.